(Adapted from the applicant's abstract) A 35-base RNA stem-loop called SL-1, located near the 5 ' end of the HIV genome, mediates several functions crucial for viral assembly, and so is a potential target for antiviral therapy. By binding the HIV Gag protein, SL1 forms part of the packaging signal that targets genomic RNA into virions. Contact between SL1 elements in a pair of HIV RNAs (through an initial "kissing-loop" complex which then converts to a linear duplex) also initiates genomic dimerization--a process that is facilitated by Gag and is essential for full infectivity. All biological activities of SL1 depend upon its three- dimensional structure, whose exact nature is unknown. The PIs laboratory has used NMR spectroscopy to examine a truncated form of SL1 in its kissing-loop and linear duplex conformations. They have obtained excellent NMR spectra on the RNA. They propose to solve the structures of the monomer and both dimer forms of this RNA, and so to obtain the first high- resolution images of a retroviral dimer interface. Guided by these structures, they will systematically mutagenize SL1 to map the molecular contacts needed to stabilize each conformational state. They also propose to use real-time NMR kinetic studies to study the sequence of events in SL1 dimerization. Multidimensional NMR on heteronuclear labeled RNA will be used to solve the structure of a full-length SL1 analogue that is fully competent for Gag binding as well as dimerization. Studies of this RNA in complex with unlabeled Gag protein will reveal the structural basis for Gag recognition and the mechanism by which Gag facilitates SL1-mediated dimerization. Results of these studies may provide a basis for rational design of new antivirals that target the SL1/Gag interaction.